The invention concerns an apparatus and a method of sieving, grading, sifting, filtering or sorting dry solid substances or solid substances in liquids.
It is known that the oscillating movements generated by ultrasound, particularly in sifting and fine sieving operations, exert an advantageous influence on the sieve throughput and the material being sieved; the material being sieved is treated carefully by the transmitted oscillating movements, in the micron range, the agglomeration forces and surface tensions are considerably reduced and the general tendency to clogging of the sieving machines decreases or is entirely eliminated.
EP 0 369 572 A2 discloses a sieving apparatus with a sieving surface which is clamped in a frame, and a piezoelectric transducer which is coupled to that surface. The transducer is a multi-part component and is clamped between two bodies of different masses, of which the one which is nearer to the sieving surface comprises two parts; one part is glued to the sieve while the other is interchangeable. Also described is a regulating circuit which is intended to keep the sieve mesh under load in a resonance condition, that however is thought not to be possible from a technical point of view.
The ultrasonic transducer is referred to as a composite transducer in the book `Ultraschall` [Ultrasound] by Wilhelm Lehfeldt 1973, Vogel-Verlag, ISBN 3-8023-0060-2, page 40, and can be employed in all areas of active ultrasound uses such as for example ultrasound welding and ultrasound cleaning. Page 48 of that book mentions circuits with automatic frequency tracking or adaptation of the control circuit indicated in EP 0 369 572 A2.
All previously known methods suffer from the disadvantage that sound distribution over the entire active sieve mesh, which in most cases comprises a wire mesh, is very poor. The reason for that characteristic is that the sieve mesh, which is generally mounted tautly in a round or rectangular steel frame, cannot be excited in a resonance condition at the working frequency. The physical requirements for that purpose do not exist. The sieve mesh can serve only as a relatively poor ultrasound conductor. If in addition material to be sieved lies on the mesh, the resulting damping effect results in a further reduction in sound conductivity.
Those interrelations mean that after just a few centimeters in distance from the sound source, the level of intensity of the sound, and therewith also the conveyor effect, is greatly reduced.
In consideration of those factors the inventor set himself the aim of substantially eliminating those disadvantages by means of a suitable design configuration.